The art of multilayer coating has been highly developed, particularly in the manufacture of photographic materials which comprise a plurality of hydrophilic layers of different composition on a hydrophobic support.
These compositions are commonly diluted with a low temperature boiling solvent, such as water, for reducing their viscosity and improving coating speed, and they are coated with a multilayer slide bead coater, multilayer cascade coater, extrusion coater or the like onto a hydrophobic support. This coating operation is followed by a drying process in which the solvent is removed.
U.S. Pat. No. 2,761,791 describes a method of multilayer bead coating whereby a plurality of liquid coating compositions are simultaneously applied to a moving support while maintaining a distinct layer relationship. During said coating method, to avoid intermixing of the composition among the lowermost layer and the layer immediately above, it is ordinarily necessary to form the lowermost layer from a coating composition of low viscosity and of a substantial thickness so that vortical action, taking place within the coating bead, is retained entirely within the lowermost layer. However, this method with said low viscosity composition can be disadvantageous since a thick layer of low viscosity coating composition comprises a large amount of water which must be removed in drying process. It may then be necessary to operate at an undesirable low speed in order not to exceed the drying capacity of commercially practical drying equipment.
U.S. Pat. No. 4,001,024 describes an improved process of multilayer bead coating wherein the lowermost layer is coated over a support as a thin layer formed from a low viscosity coating composition and the layer immediately above is coated over said lowermost layer as a thicker layer of higher viscosity so that vortical action of the coating bead is confined to the lowermost layer and the layer immediately above it, while all other layers are coated in discrete form (not affected by the vortex). According to the aforesaid process, some inter-mixing results between the lowermost layer and the layer immediately above it so that is necessary to choose particular compositions to assure that this interlayer mixing is not harmful to the product.
A homogeneous coating quality is important for the production of high quality photographic materials, especially for the reproduction of image areas of uniform density. This can easily be achieved using multilayer bead coating techniques, if support materials of uniform thickness are available. The thickness of the multilayer liquid film, formed on the slide of the coating bar, remains uniform even after application onto the support. The photographic material is then set by chilling the whole system, whereby the uniform structure of the photographic layers is frozen in.
The surface of support material is uneven. However the multilayer liquid coatings applied on such a support tend to even out the irregular surface structure. If the aforesaid support has an uneven surface, it may provide an evident density variation pattern on the finished photographic material. In fact as soon as the low viscosity coating composition is laid down on the moving support, it copies any support pattern due to the residual running propensity of the coating composition before the chilling takes place and this pattern is copied by the upper layers.
The action of surface tension and gravity forces in the chilling zone therefore produces an irregular thickness profile of the photographic emulsion layers, which is frozen in after setting and can be seen as a mottled structure (particularly uneven coating formed in a direction orthogonal or parallel to the coating direction) in the final processed image. The resulting image likewise contains all the variations of the layer thickness induced by the structure surface of the support.
The time scale for this hydrodynamic process depends on the viscosity of the coating compositions, but for the range of 10-30 mPa/s (milliPascal per second), a half-life of 0.2 to 1 sec can be assumed. This process therefore is much faster than the concurrent viscosity increase of the coated liquid layers in the chilling bed, as described in the Research Disclosure 24844, December 1984.
Although various methods of improving coating quality have been described in the art, such as (1) the application of a set undercoat, on which the further liquid emulsion layers are applied at a second coating pass, or (2) the slowing down of the hydrodynamic processes through improved rheological properties of the coating solution, or (3) the control of the temperature in the coating zone, only marginal benefits can be gained in these ways. An example of method (1) is described in British Pat. No. 855.849. Examples of method (2) are described in U.S. Pat. No(s). 4,113,903 and 4,525,392. An example of method (3) is described in U.S. Pat. No. 4,051,278.